Use "low key" terminology in nbtsort.c.

nbtree index builds once stashed the "minimum key" for a page, which was
used as the basis of the pivot tuple that gets placed in the next level
up (i.e. the tuple that stores the downlink to the page in question).
It doesn't quite work that way anymore, so the "minimum key" terminology
now seems misleading (these days the minimum key is actually a straight
copy of the high key from the left sibling, which is a distinct thing in
subtle but important ways).  Rename this concept to "low key".  This
name is a lot clearer given that there is now a sharp distinction
between pivot and non-pivot tuples.  Also remove comments that describe
obsolete details about how the minimum key concept used to work.

Rather than generating the minus infinity item for the leftmost page on
a level by copying the new item and truncating that copy, simply
allocate a small buffer.  The old approach confusingly created the
impression that the new item had some kind of significance.  This was
another artifact of how things used to work before commits 8224de4f and
dd299df8.

src/backend/access/nbtree/nbtsort.c

@@ -236,21 +236,12 @@ typedef struct BTBuildState
 /*
  * Status record for a btree page being built.  We have one of these
  * for each active tree level.
- *
- * The reason we need to store a copy of the minimum key is that we'll
- * need to propagate it to the parent node when this page is linked
- * into its parent.  However, if the page is not a leaf page, the first
- * entry on the page doesn't need to contain a key, so we will not have
- * stored the key itself on the page.  (You might think we could skip
- * copying the minimum key on leaf pages, but actually we must have a
- * writable copy anyway because we'll poke the page's address into it
- * before passing it up to the parent...)
  */
 typedef struct BTPageState
 {
 	Page		btps_page;		/* workspace for page building */
 	BlockNumber btps_blkno;		/* block # to write this page at */
-	IndexTuple	btps_minkey;	/* copy of minimum key (first item) on page */
+	IndexTuple	btps_lowkey;	/* page's strict lower bound pivot tuple */
 	OffsetNumber btps_lastoff;	/* last item offset loaded */
 	uint32		btps_level;		/* tree level (0 = leaf) */
 	Size		btps_full;		/* "full" if less than this much free space */
@@ -717,7 +708,7 @@ _bt_pagestate(BTWriteState *wstate, uint32 level)
 	/* and assign it a page position */
 	state->btps_blkno = wstate->btws_pages_alloced++;
 
-	state->btps_minkey = NULL;
+	state->btps_lowkey = NULL;
 	/* initialize lastoff so first item goes into P_FIRSTKEY */
 	state->btps_lastoff = P_HIKEY;
 	state->btps_level = level;
@@ -980,28 +971,27 @@ _bt_buildadd(BTWriteState *wstate, BTPageState *state, IndexTuple itup)
 		}
 
 		/*
-		 * Link the old page into its parent, using its minimum key. If we
-		 * don't have a parent, we have to create one; this adds a new btree
-		 * level.
+		 * Link the old page into its parent, using its low key.  If we don't
+		 * have a parent, we have to create one; this adds a new btree level.
 		 */
 		if (state->btps_next == NULL)
 			state->btps_next = _bt_pagestate(wstate, state->btps_level + 1);
 
-		Assert((BTreeTupleGetNAtts(state->btps_minkey, wstate->index) <=
+		Assert((BTreeTupleGetNAtts(state->btps_lowkey, wstate->index) <=
 				IndexRelationGetNumberOfKeyAttributes(wstate->index) &&
-				BTreeTupleGetNAtts(state->btps_minkey, wstate->index) > 0) ||
+				BTreeTupleGetNAtts(state->btps_lowkey, wstate->index) > 0) ||
 			   P_LEFTMOST((BTPageOpaque) PageGetSpecialPointer(opage)));
-		Assert(BTreeTupleGetNAtts(state->btps_minkey, wstate->index) == 0 ||
+		Assert(BTreeTupleGetNAtts(state->btps_lowkey, wstate->index) == 0 ||
 			   !P_LEFTMOST((BTPageOpaque) PageGetSpecialPointer(opage)));
-		BTreeInnerTupleSetDownLink(state->btps_minkey, oblkno);
-		_bt_buildadd(wstate, state->btps_next, state->btps_minkey);
-		pfree(state->btps_minkey);
+		BTreeInnerTupleSetDownLink(state->btps_lowkey, oblkno);
+		_bt_buildadd(wstate, state->btps_next, state->btps_lowkey);
+		pfree(state->btps_lowkey);
 
 		/*
-		 * Save a copy of the high key from the old page.  It is also used as
-		 * the minimum key for the new page.
+		 * Save a copy of the high key from the old page.  It is also the low
+		 * key for the new page.
 		 */
-		state->btps_minkey = CopyIndexTuple(oitup);
+		state->btps_lowkey = CopyIndexTuple(oitup);
 
 		/*
 		 * Set the sibling links for both pages.
@@ -1032,18 +1022,17 @@ _bt_buildadd(BTWriteState *wstate, BTPageState *state, IndexTuple itup)
 	 * was created that became the current page.  Either way, the current page
 	 * definitely has space for new item.
 	 *
-	 * If the new item is the first for its page, stash a copy for later. Note
-	 * this will only happen for the first item on a level; on later pages,
-	 * the first item for a page is copied from the prior page in the code
-	 * above.  The minimum key for an entire level is nothing more than a
-	 * minus infinity (downlink only) pivot tuple placeholder.
+	 * If the new item is the first for its page, it must also be the first
+	 * item on its entire level.  On later same-level pages, a low key for a
+	 * page will be copied from the prior page in the code above.  Generate a
+	 * minus infinity low key here instead.
 	 */
 	if (last_off == P_HIKEY)
 	{
-		Assert(state->btps_minkey == NULL);
-		state->btps_minkey = CopyIndexTuple(itup);
-		/* _bt_sortaddtup() will perform full truncation later */
-		BTreeTupleSetNAtts(state->btps_minkey, 0);
+		Assert(state->btps_lowkey == NULL);
+		state->btps_lowkey = palloc0(sizeof(IndexTupleData));
+		state->btps_lowkey->t_info = sizeof(IndexTupleData);
+		BTreeTupleSetNAtts(state->btps_lowkey, 0);
 	}
 
 	/*
@@ -1083,7 +1072,7 @@ _bt_uppershutdown(BTWriteState *wstate, BTPageState *state)
 		 * We have to link the last page on this level to somewhere.
 		 *
 		 * If we're at the top, it's the root, so attach it to the metapage.
-		 * Otherwise, add an entry for it to its parent using its minimum key.
+		 * Otherwise, add an entry for it to its parent using its low key.
 		 * This may cause the last page of the parent level to split, but
 		 * that's not a problem -- we haven't gotten to it yet.
 		 */
@@ -1095,16 +1084,16 @@ _bt_uppershutdown(BTWriteState *wstate, BTPageState *state)
 		}
 		else
 		{
-			Assert((BTreeTupleGetNAtts(s->btps_minkey, wstate->index) <=
+			Assert((BTreeTupleGetNAtts(s->btps_lowkey, wstate->index) <=
 					IndexRelationGetNumberOfKeyAttributes(wstate->index) &&
-					BTreeTupleGetNAtts(s->btps_minkey, wstate->index) > 0) ||
+					BTreeTupleGetNAtts(s->btps_lowkey, wstate->index) > 0) ||
 				   P_LEFTMOST(opaque));
-			Assert(BTreeTupleGetNAtts(s->btps_minkey, wstate->index) == 0 ||
+			Assert(BTreeTupleGetNAtts(s->btps_lowkey, wstate->index) == 0 ||
 				   !P_LEFTMOST(opaque));
-			BTreeInnerTupleSetDownLink(s->btps_minkey, blkno);
-			_bt_buildadd(wstate, s->btps_next, s->btps_minkey);
-			pfree(s->btps_minkey);
-			s->btps_minkey = NULL;
+			BTreeInnerTupleSetDownLink(s->btps_lowkey, blkno);
+			_bt_buildadd(wstate, s->btps_next, s->btps_lowkey);
+			pfree(s->btps_lowkey);
+			s->btps_lowkey = NULL;
 		}
 
 		/*